Support External CI for Merge Request verification

Description

Gitlab has working support for CI testing via external framework via status API. I am working in integrating the support for merge_request event in the Buildbot CI framework. https://github.com/buildbot/buildbot/pull/3291 The event are properly working, but the results are not properly propagated to the merge request status page.

It is important for larger scale users or people with a more complex pipeline to be able to use external CI framework, and pre-submit CI is an important feature to have.

Looking at the source code of the merge_request page, it looks like the page is directly looking at the gitlab pipeline report, and thus is only integrated with that.

Proposal

I am not exactly sure how to resolve this problem. I think the most simple way to do it is to detect the use of refs/merge-requests/1/head in commit status API, and build a fake merge_request pipeline status in that case.